Zoom lens and image pickup apparatus having the same

ABSTRACT

A zoom lens includes, in order from an object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and three or more lens units. A distance between adjacent lens units varies during zooming. A predetermined condition is satisfied.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to zoom lens, which is suitable for animage pickup apparatus, such as a digital video camera, a digital stillcamera, a broadcasting camera, a film-based camera, and a surveillancecamera.

Description of the Related Art

Along with the increasing functionality of image pickup apparatuses,there has recently been a demand for a compact and lightweight zoom lenshaving a large aperture ratio. One known zoom lens that satisfies thisrequirement is a negative lead type zoom lens in which a lens unithaving a negative refractive power is disposed closest to the object.

Japanese Patent No. (“JP”) 4984231 discloses a zoom lens that includes,in order from an object side to an image side, first to fifth lens unitshaving negative, positive, negative, positive, and positive refractivepowers, wherein the first lens unit moves to the image side and thenmoves to the object side during zooming.

JP 5891860 discloses a mirrorless type zoom lens that includes, in orderfrom the object side to the image side, first to fourth lens unitshaving negative, positive, negative, and positive refractive powers.

In the zoom lens disclosed in JP 4984231, the first lens unit has astrong refractive power and draws a substantially reciprocating locusduring zooming, so that the overall lens length at the telephoto endbecomes long. In addition, this zoom lens has a long backfocus, and itis difficult to reduce the overall lens length. Further, a long distanceon the optical axis from a lens surface closest to the object to a lenssurface closest to an image plane in the first lens unit, which isnecessary to correct the aberration in the first lens unit, is notsuitable for the miniaturization and weight reduction.

The zoom lens disclosed in JP 5891860 has a four-unit structure, and ifa larger aperture ratio is designed, various aberrations at thetelephoto end and aberrational fluctuations during zooming cannot besuppressed, and it is difficult to realize a high optical performance inthe overall zoom range.

SUMMARY OF THE INVENTION

The present invention provides a compact zoom lens having a largeaperture ratio and a high optical performance, and an image pickupapparatus having the same.

A zoom lens according to one aspect of the present invention includes,in order from an object side to an image side, a first lens unit havinga negative refractive power, a second lens unit having a positiverefractive power, and three or more lens units. A distance betweenadjacent lens units varies during zooming. The following conditionalexpressions are satisfied:

4.8<m1·f1/skw ²<15.0

2.45<β2t/β2w<50.00

where m1 is a moving amount of the first lens unit during zooming from awide-angle end to a telephoto end, f1 is a focal length of the firstlens unit, skw is a backfocus at the wide-angle end, β2t is a lateralmagnification of the second lens unit at the telephoto end, and β2w is alateral magnification of the second lens unit at the wide-angle end.

An image pickup apparatus having the above zoom lens also constitutesanother aspect of the present invention.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a zoom lens according to Example 1 at awide-angle end.

FIGS. 2A to 2C are longitudinal aberration diagrams of the zoom lensaccording to Example 1 at the wide-angle end (FIG. 2A), a middle zoomposition (FIG. 2B), and a telephoto end (FIG. 2C).

FIG. 3 is a sectional view of a zoom lens according to Example 2 at awide-angle end.

FIGS. 4A to 4C are longitudinal aberration diagrams of the zoom lensaccording to Example 2 at the wide-angle end (FIG. 4A), a middle zoomposition (FIG. 4B), and a telephoto end (FIG. 4C).

FIG. 5 is a sectional view of a zoom lens according to Example 3 at awide-angle end.

FIGS. 6A to 6C are longitudinal aberration diagrams of the zoom lensaccording to Example 3 at the wide-angle end (FIG. 6A), a middle zoomposition (FIG. 6B), and a telephoto end (FIG. 6C).

FIG. 7 is a sectional view of a zoom lens according to Example 4 at awide-angle end.

FIGS. 8A to 8C are longitudinal aberration diagrams of the zoom lensaccording to Example 4 at the wide-angle end (FIG. 8A), a middle zoomposition (FIG. 8B), and a telephoto end (FIG. 8C).

FIG. 9 is a sectional view of a zoom lens according to Example 5 at awide-angle end.

FIGS. 10A to 10C are aberration diagrams of the zoom lens according toExample 5 at the wide-angle end (FIG. 10A), a middle zoom position (FIG.10B), and a telephoto end (FIG. 10C).

FIG. 11 is a schematic view of an image pickup apparatus.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the accompanying drawings, a detailed description willbe given of embodiments according to the present invention.Corresponding elements in respective figures will be designated by thesame reference numerals, and a duplicate description thereof will beomitted.

FIGS. 1, 3, 5, 7, and 9 are sectional views of zoom lenses according toExamples 1 to 5, respectively, at the wide-angle end. The zoom lensaccording to each example is used in an image pickup apparatus such as adigital video camera, a digital still camera, a broadcasting camera, afilm-based camera, and a surveillance camera. Further, the zoom lensaccording to each example can also be used as a projection opticalsystem for a projection apparatus (projector).

In each sectional view, the left side is the object side and the rightside is the image side. The zoom lens according to each example includesa plurality of lens units. In this specification, a lens unit is a groupof lenses that move or stand still integrally during zooming. That is,in the zoom lenses according to each example, a distance betweenadjacent lens units varies during zooming. An arrow shown in eachsectional view indicates a moving direction of a lens unit duringzooming from the wide-angle end to the telephoto end. An arrow withFocus indicates a moving direction of a focus unit during focusing froman object at infinity (infinity object) to a short-distance object. Thelens unit may include one or more lenses. The lens unit may include adiaphragm (aperture stop).

The zoom lenses according to each example includes, in order from theobject side to the image side, a first lens unit L1 having a negativerefractive power, a second lens unit L2 having a positive refractivepower, and three or more subsequent lens units. A structure on the imageside of the second lens unit L2, which has a thicker light beam width,has a multi-unit structure with three or more lens units, and each lensunit moves with an independent trajectory during zooming so as tosuppress fluctuations of the spherical aberration and coma duringzooming in a large-diameter zoom lens.

In each sectional view, Li represents an i-th lens unit (where i is anatural number) counted from the object side among the lens unitsincluded in the zoom lens. SP represents the diaphragm (aperture stop).IP represents an image plane, and when the zoom lens according to eachexample is used as an imaging optical system of a digital still cameraor a digital video camera, the imaging surface of a solid-state imagesensor (photoelectric conversion element) such as a CCD sensor and aCMOS sensor is placed there. When the zoom lens according to eachexample is used as an imaging optical system of the film-based camera, aphotosensitive surface corresponding to a film plane is placed on theimage plane IP.

FIGS. 2A to 2C, 4A to 4C, 6A to 6C, 8A to 8C, 10A to 10C arelongitudinal aberration diagrams of the zoom lenses according toExamples 1 to 5, respectively. In each aberration diagram, FIGS. 2A, 4A,6A, 8A, and 10A are longitudinal aberration diagrams at the wide-angleend, FIGS. 2B, 4B, 6B, 8B, and 10B are longitudinal aberration diagramsat a middle zoom position, and FIGS. 2C, 4C, 6C, 8C, and 10C arelongitudinal aberration diagrams at the telephoto end.

In a spherical aberration diagram, Fno represents an F-number and showsspherical aberration amounts for the d-line (wavelength 587.6 nm) andthe g-line (wavelength 435.8 nm). In a astigmatism diagram, ΔSrepresents an astigmatism amount on a sagittal image plane for thed-line, and ΔM represents an astigmatism amount on a meridional imageplane for the d-line. A distortion diagram shows a distortion amount forthe d-line. A chromatic aberration diagram shows a chromatic aberrationamount for the g-line. ω is a half angle of view (degrees).

Next follows a description of a characteristic structure of the zoomlens according to each example.

In the zoom lens according to each example, the distance betweenadjacent lens units varies during zooming from the wide-angle end to thetelephoto end. In each example, the zoom locus is set so that theoverall length of the zoom lens (overall lens length) is the shortest atthe telephoto end. During zooming, the main magnification variation isperformed by greatly reducing the distance between the first lens unitL1 and the second lens unit L2. In particular, the second lens unit L2has a strong refractive power and a large magnification varying effectwhen it is significantly moved to the object side during zooming.

The zoom lens according to each example satisfies the followingconditional expression (1):

4.8<m1·f1/skw ²<15.0  (1)

where m1 is a moving amount (zoom moving amount) of the first lens unitL1 during zooming from the wide-angle end to the telephoto end, f1 is afocal length of the first lens unit L1, and skw is a backfocus at thewide-angle end. In this example, the moving amount of the lens unitcorresponds to a difference between the position on the optical axis atthe wide-angle end and the position on the optical axis at the telephotoend. The sign of the moving amount is set negative when the lens unit islocated on the image side at the telephoto end relative to its positionat the wide-angle end, and positive when the lens unit is located on theobject side at the telephoto end relative to its position at thewide-angle end.

The conditional expression (1) defines a relationship among the zoommoving amount m1 of the first lens unit L1, the focal length f1 of thefirst lens unit L1, and the backfocus skw at the wide-angle end, inorder to reduce the overall lens length at the wide-angle end and tocorrect the aberrations in the overall zoom range.

If the zoom moving amount m1 of the first lens unit L1 becomes largerand the value becomes higher than the upper limit in the conditionalexpression (1), the overall lens length at the wide-angle end becomeslong and the front lens diameter also becomes large. If the focal lengthf1 of the first lens unit L1 becomes longer and the value becomes higherthan the upper limit in the conditional expression (1), the refractivepower of the first lens unit L1 becomes weaker, and the negativerefractive power arrangement of the retrofocus at the wide-angle endweakens. This may prevent a short overall lens length. If the backfocusskw at the wide-angle end becomes smaller and the value becomes higherthan the upper limit in the conditional expression (1), the mechanicallayout of the connector between the lens and the camera becomesdifficult.

If the zoom moving amount m1 of the first lens unit L1 becomes smallerand the value becomes lower than the lower limit in the conditionalexpression (1), it becomes difficult to obtain a desired zoommagnification. If the focal length f1 of the first lens unit L1 becomesshorter and the value becomes lower than the lower limit in theconditional expression (1), it is advantageous for a short overall lenslength at the wide-angle end and a small front lens diameter but it isdifficult to correct the lateral chromatic aberration and the distortiongenerated in the first lens unit L1 using the subsequent lens units. Ifthe backfocus skw at the wide-angle end becomes larger and the valuebecomes lower than the lower limit in the conditional expression (1),reducing the overall lens length may be prevented.

The zoom lens according to each example satisfies the followingconditional expression (2):

2.45<β2t/β2w<50.00  (2)

where β2t is a lateral magnification of the second lens unit L2 at thetelephoto end and β2w is a lateral magnification of the second lens unitL2 at the wide-angle end.

The conditional expression (2) defines a ratio of the lateralmagnification β2t of the second lens unit L2 at the telephoto end to thelateral magnification β2w of the second lens unit L2 at the wide-angleend or a so-called magnification varying ratio of second lens unit L2,in order to reduce the overall lens length and the diameter of the lensbarrel.

If the magnification varying ratio of the second lens unit L2 is higherthan the upper limit in the conditional expression (2), it isadvantageous for increasing the magnification and for reducing the lensdiameters of the subsequent lens units. However, it is difficult tosuppress the spherical aberration, the coma, and the longitudinalchromatic aberration generated in the second lens unit L2.

If the magnification varying ratio of the second lens unit L2 is smallerand the value becomes lower than the lower limit in the conditionalexpression (2), it is necessary to increase the refractive powers andzoom moving amounts of the first lens unit L1 and the subsequent lensunits in order to obtain a desired zoom magnification and this mayprevent a reduced overall lens length and an improved opticalperformance.

The zoom lens according to each example may further satisfy thefollowing conditional expression (3):

0.05<TD1/ft<0.50  (3)

where TD1 is a distance on the optical axis (overall thickness of thefirst lens unit L1) from a lens surface closest to the object to a lenssurface closest to the image plane in the first lens unit L1, and ft isa focal length of the zoom lens at the telephoto end.

The conditional expression (3) defines a relationship between theoverall thickness TD1 of the first lens unit L1 and the focal length ftof the zoom lens at the telephoto end, in order to reduce the overalllens length and the front lens diameter.

If the overall thickness TD1 of the first lens unit L1 becomes longerand the value becomes higher than the upper limit in the conditionalexpression (3), the overall lens length at the wide-angle end becomeslonger and the front lens diameter becomes larger. If the focal lengthft of the zoom lens at the telephoto end becomes shorter and the valuebecomes higher than the upper limit in the conditional expression (3), asufficient focal length and zoom magnification at the telephoto end areunavailable.

If the overall thickness TD1 of the first lens unit L1 becomes shorterand the value becomes lower than the lower limit in the conditionalexpression (3), it is advantageous for the reduced overall lens lengthand front lens diameter at the wide-angle end but it becomes difficultto arrange refractive powers so as to correct the curvature of field andthe lateral chromatic aberration generated in the first lens unit L1 atthe wide-angle end and it becomes difficult to maintain the opticalperformance. If the focal length ft of the zoom lens at the telephotoend becomes longer and the value becomes lower than the lower limit inthe conditional expression (3), a desired angle of view is available,but it becomes difficult to sufficiently correct the longitudinalchromatic aberration and spherical aberration generated at the telephotoend.

The zoom lens according to each example may further satisfy thefollowing conditional expression (4):

−4.0<f1/fw<−1.0  (4)

where fw is a focal length of the zoom lens at the wide-angle end.

The conditional expression (4) defines a relationship between the focallength f1 of the first lens unit L1 and the focal length fw of the zoomlens at the wide-angle end in order to obtain the required zoom ratio.

If the focal length f1 of the first lens unit L1 becomes shorter and thevalue becomes higher than the upper limit in the conditional expression(4), the zoom lens can be easily made smaller but it is difficult tosatisfactorily correct the distortion and lateral chromatic aberrationat the wide-angle end with a small number of lenses. If the focal lengthfw of the zoom lens at the wide-angle end is longer and the valuebecomes higher than the upper limit in the conditional expression (4),it becomes difficult to secure a desired zoom ratio.

If the focal length f1 of the first lens unit L1 becomes longer and thevalue becomes lower than the lower limit in the conditional expression(4), it becomes easier to correct the lateral chromatic aberration atthe wide-angle end and the longitudinal chromatic aberration at thetelephoto end, but the zoom moving amount m1 of the first lens unit L1becomes large and the overall lens length becomes long. If the focallength fw of the zoom lens at the wide-angle end becomes shorter and thevalue becomes lower than the lower limit in the conditional expression(4), the front lens diameter becomes larger as the wide-angle schemeproceeds.

The zoom lens according to each example may further satisfy thefollowing conditional expression (5):

0.8<f2/fw<4.0  (5)

where f2 is a focal length of the second lens unit L2.

The conditional expression (5) defines a relationship between the focallength f2 of the second lens unit L2 and the focal length fw of the zoomlens at the wide-angle end, in order to reduce the lens diameter and torealize a high optical performance in the overall zoom range.

If the focal length f2 of the second lens unit L2 becomes longer and thevalue becomes higher than the upper limit in the conditional expression(5), it becomes difficult to sufficiently reduce the lens diameter ofthe subsequent lens units. If the focal length fw of the zoom lens atthe wide-angle end is longer and the value becomes higher than the upperlimit in the conditional expression (5), the front lens diameter becomeslarge due to the wide-angle scheme.

When the focal length f2 of the second lens unit L2 becomes shorter andthe value is lower than the lower limit in the conditional expression(5), it is advantageous for the reduced lens diameter but it becomesdifficult to sufficiently correct the spherical aberration generated inthe second lens unit L2. If the focal length fw of the zoom lens at thewide-angle end becomes longer and the value becomes lower than the lowerlimit in the conditional expression (5), a desired angle of view cannotbe obtained at the wide-angle end.

The zoom lens according to each example may further satisfy thefollowing conditional expression (6):

0.3<|fr|/fw<20.0  (6)

where fr is a focal length of the lens unit (final lens unit) closest tothe image plane.

The conditional expression (6) defines a relationship between the focallength fr of the final lens unit and the focal length fw of the zoomlens at the wide-angle end, in order to properly correct the curvatureof field and to properly set the light incident angle on the imageplane.

If the focal length fr of the final lens unit becomes longer and thevalue becomes higher than the upper limit in the conditional expression(6), the refractive power of the final lens unit becomes too weak, andit becomes difficult to provide a sufficient curvature of fieldaberration effect. Further, it is necessary to secure a space fordisposing a lens unit having no refractive power, and theminiaturization may be hindered. Moreover, if the focal length fw of thezoom lens at the wide-angle end is longer and the value becomes higherthan the upper limit in the conditional expression (6), the front lensdiameter becomes large due to the wide-angle scheme.

If the focal length fr of the final lens unit becomes shorter and thevalue becomes lower than the lower limit in the conditional expression(6), the effect of reducing the overall lens length becomes stronger,but the absolute value of the light incident angle on the image planebecomes too large and shading generated in the image sensor tends to belarge. Further, the curvature of field correction effect in the finallens unit becomes excessive.

The zoom lens according to each example may further satisfy thefollowing conditional expression (7):

0.5<POw/fw<3.0  (7)

where POw is a distance from the image plane to the exit pupil positionat the wide-angle end. The sign of the distance from the image plane tothe exit pupil position is negative when the exit pupil position islocated on the object side of the image plane and positive when it islocated on the image side of the image plane.

The conditional expression (7) defines a relationship between thedistance POw from the image plane to the exit pupil position at thewide-angle end and the focal length fw of the zoom lens at thewide-angle end, in order to secure a high telecentricity, to properlycorrect the curvature of field, and to properly set the light incidentangle on the image plane.

If the distance POw from the image plane to the exit pupil position atthe wide-angle end becomes shorter and the value becomes higher than theupper limit in the conditional expression (7), the light incident angleon the image plane at the peripheral image height becomes too high andmay cause so-called shading. If the focal length fw of the zoom lens atthe wide-angle end is shorter and the value becomes higher than theupper limit in the conditional expression (7), it becomes difficult toachieve a desired zoom magnification.

When the distance POw from the image plane to the exit pupil position atthe wide-angle end becomes longer and the value becomes lower than thelower limit in the conditional expression (7), the refractive power ofthe final lens unit tends to increase, and it becomes difficult tosufficiently suppress the curvature of field.

The zoom lens according to each example may further satisfy thefollowing conditional expression (8) wherein first lens unit L1 includesa negative lens closest to the object:

−3.0<(R2+R1)/(R2−R1)<−0.3  (8)

where R1 is a radius of curvature of the negative lens on the objectside, and R2 is a radius of curvature of the negative lens on the imageside.

The conditional expression (8) defines a shape of the negative lensclosest to the object in the first lens unit L1 or a so-called shapefactor of the negative lens, in order to achieve both a short lensdiameter of the front lens and suppressed distortion.

If the biconcave shape of the negative lens becomes stronger and thevalue becomes higher than the upper limit in the conditional expression(8), a radius of curvature R1 on the object side becomes too strong andthe distortion becomes too strong. Further, when the biconcave shapebecomes stronger, the refractive power of the negative lens also becomesstronger, and it becomes difficult to suppress the lateral chromaticaberration and curvature of field at the wide-angle end.

If the shape factor of the negative lens becomes negatively larger andthe value becomes lower than the lower limit in the conditionalexpression (8), the negative meniscus shape becomes too strong in whichthe lens shape is convex toward the object. If the negative meniscusshape becomes too strong, it is advantageous for suppressing thedistortion, but the overall thickness of the first lens unit L1increases and the diameter of the mechanical member at the tip of thezoom lens increases and prevents the miniaturization.

The zoom lens according to each example may further satisfy thefollowing conditional expression (9):

0.2<m2/fw<1.4  (9)

where m2 is a moving amount of the second lens unit L2 during zoomingfrom the wide-angle end to the telephoto end.

The conditional expression (9) defines a relationship between the zoommoving amount m2 of the second lens unit L2 and the focal length of thezoom lens at the wide-angle end, in order to determine a proper zoommoving amount of the second lens unit L2.

If the zoom moving amount m2 of the second lens unit L2 becomes largerand the value becomes higher than the upper limit in the conditionalexpression (9), it becomes difficult to reduce the overall lens lengthat the telephoto end. Further, as the zoom moving amount m2 becomeslarger, a ratio change of a vertical width of the luminous flux betweenthe wide-angle end and the telephoto end becomes large. As a result, adecrease of a peripheral light amount generated when the luminous fluxis narrowed by the diaphragm becomes remarkable. Further, if the focallength fw of the zoom lens at the wide-angle end is shorter and thevalue becomes higher than the upper limit in the conditional expression(9), the front lens diameter becomes larger due to the wide-anglescheme.

When the zoom moving amount m2 of the second lens unit L2 becomessmaller and the value becomes lower than the lower limit in theconditional expression (9), it is necessary to extend the focal lengthf2 of the second lens unit L2 which is a main magnification varying lensunit, and it consequently becomes difficult to suppress the sphericalaberration generated at the telephoto end. If the focal length fw of thezoom lens at the wide-angle end becomes longer and the value becomeslower than the lower limit in the conditional expression (9), thedesired zoom ratio cannot be achieved.

The zoom lens may further satisfy the following conditional expression(10):

−3.0<fg1/fw<−0.5  (10)

where fg1 is a focal length of a lens closest to the object in the zoomlens.

The conditional expression (10) defines a relationship between the focallength fg1 of the lens closest to the object in the zoom lens and thefocal length fw of the zoom lens at the wide-angle end, in order toachieve both a good optical performance and the miniaturization at thewide-angle end.

If the focal length fg1 of the lens closest to the object in the zoomlens is shorter and the value becomes higher than the upper limit in theconditional expression (10), the retrofocus arrangement becomes strong,which is advantageous for reducing the overall lens length at thewide-angle end but the distortion cannot be sufficiently suppressed.Further, if the focal length fw of the zoom lens at the wide-angle endbecomes longer and the value becomes higher than the upper limit in theconditional expression (10), a desired angle of view at the wide-angleend cannot be obtained.

If the focal length fg1 of the lens closest to the object in the zoomlens is longer and the value becomes lower than the lower limit in theconditional expression (10), the retrofocus arrangement at thewide-angle end becomes weaker and may increase the overall lens lengthand the front lens diameter. If the focal length fw of the zoom lens atthe wide-angle end is shorter and the value becomes lower than the lowerlimit in the conditional expression (10), the front lens diameterbecomes larger due to the wide-angle scheme.

The zoom lens may further satisfy the following conditional expression(11):

3.5<TTDw/skw<22.0  (11)

where TTDw is an overall lens length at the wide-angle end.

The conditional expression (11) defines a relationship between theoverall lens length TTDw and the backfocus skw at the wide-angle end inorder to obtain a zoom lens with a small overall lens length.

If the overall lens length TTDw at the wide-angle end increases and thevalue becomes higher than the upper limit in the conditional expression(11), a reduced overall lens length cannot be met. Further, if thebackfocus ski becomes smaller and the value becomes higher than theupper limit in the conditional expression (11), the mechanical layout ofthe connector between the lens and the camera becomes difficult.

If the overall lens length at the wide-angle end becomes smaller and thevalue becomes lower than the lower limit in the conditional expression(11), the positive refractive power of the entire lens becomes too high,and it becomes difficult to control the Petzval sum and to obtain thedesired optical performance. Further, if the backfocus skew becomeslarger and the value becomes lower than the lower limit in theconditional expression (11), it becomes difficult to reduce the overalllens length.

The numerical ranges of the conditional expressions (1) to (11) may bethe numerical ranges of the following conditional expressions (1a) to(11a):

4.9<m1·f1/skw ²<14.0  (1a)

2.50<β2t/β2w<30.00  (2a)

0.08<TD1/ft<0.40  (3a)

−3.0<f1/fw<−1.2  (4a)

1.0<f2/fw<3.0  (5a)

0.5<|fr|/fw<16.0  (6a)

0.6<POw/fw<2.0  (7a)

−2.0<(R2+R1)/(R2−R1)<−0.5  (8a)

0.3<m2/fw<1.0  (9a)

−2.0<fg1/fw<−0.8  (10a)

4.0<TTDw/skw<20.0  (11a)

The numerical ranges of the conditional expressions (1) to (11) may bethe numerical ranges of the following conditional expressions (1b) to(11b):

4.9<m1·f1/skw ²<13.0  (1b)

2.50<β2t/β2w<25.00  (2b)

0.11<TD1/ft<0.30  (3b)

−2.5<f1/fw<−1.5  (4b)

1.3<f2/fw<2.8  (5b)

0.7<|fr|/fw<14.0  (6b)

0.8<POw/fw<1.7  (7b)

−1.2<(R2+R1)/(R2−R1)<−0.7  (8b)

0.4<m2/fw<0.9  (9b)

−1.7<fg1/fw<−1.1  (10b)

6.0<TTDw/skw<17.0  (11b)

In order to reduce the weight of the first lens unit L1, the number oflenses in the first lens unit L1 may be two or less. Further, the firstlens unit L1 may have two lenses having negative and positive refractivepowers arranged from the object side to the image side, in order tocorrect the chromatic aberration.

In order to perform light weight and quick focusing, focusing may beperformed on the image side of the second lens unit L2. In order tosuppress the fluctuation of the curvature of field due to focusing, thethird lens unit L3 may be moved during focusing.

A so-called floating configuration may be used that moves two or morelens units in order to suppress the fluctuations of the sphericalaberration and curvature of field during focusing.

Next follows a detailed description of the optical system according toeach example.

The zoom lens according to Example 1 is a six-unit zoom lens thatincludes, in order from the object side to the image side, a first lensunit L1 having a negative refractive power, a second lens unit L2 havinga positive refractive power, a third lens unit L3 having a negativerefractive power, a fourth lens unit L4 having a positive refractivepower, a fifth lens unit L5 having a negative refractive power, and asixth lens unit L6 having a positive refractive power. The zoom lensaccording to Example 1 is a zoom lens having a zoom ratio of 2.4 and anF-number of 2.9. During zooming from the wide-angle end to the telephotoend, the first lens unit L1 monotonically moves to the image side, andthe second lens unit L2 to the sixth lens unit L6 move to the objectside. During zooming, the second lens unit L2 and the fourth lens unitL4 move integrally (with the same trajectory) toward the object side.During focusing, the third lens unit L3 and the fifth lens unit L5 move.

The zoom lens according to Example 2 is a six-unit zoom lens thatincludes, in order from the object side to the image side, a first lensunit L1 having a negative refractive power, a second lens unit L2 havinga positive refractive power, a third lens unit L3 having a positiverefractive power, a fourth lens unit L4 having a positive refractivepower, a fifth lens unit L5 having a negative refractive power, and asixth lens unit L6 having a negative refractive power. The zoom lensaccording to Example 2 is a zoom lens having a zoom ratio of 2.4 and anF-number of 2.9. During zooming from the wide-angle end to the telephotoend, the first lens unit L1 monotonically moves to the image side, andthe second lens unit L2 to the sixth lens unit L6 move to the objectside. During zooming, the second lens unit L2 and the fourth lens unitL4 move integrally (with the same trajectory) toward the object side.During focusing, the third lens unit L3 and the fifth lens unit L5 move.

The zoom lens according to Example 3 is a five-unit zoom lens thatincludes, in order from the object side to the image side, a first lensunit L1 having a negative refractive power, a second lens unit L2 havinga positive refractive power, a third lens unit L3 having a positiverefractive power, a fourth lens unit L4 having a negative refractivepower, and a fifth lens unit L5 having a positive refractive power. Thezoom lens according to Example 3 is a zoom lens having a zoom ratio of2.1 and an F-number of 4.12. During zooming from the wide-angle end tothe telephoto end, the first lens unit L1 monotonically moves to theimage side, and the second lens unit L2 to the fifth lens unit L5 moveto the object side. During focusing, the fourth lens unit L4 moves.

The zoom lenses according to Example 4 is a five-unit zoom lens thatincludes, in order from the object side to the image side, a first lensunit L1 having a negative refractive power, a second lens unit L2 havinga positive refractive power, a third lens unit L3 having a positiverefractive power, a fourth lens unit L4 having a negative refractivepower, and a fifth lens unit L5 having a negative refractive power. Thezoom lens according to Example 4 is a zoom lens having a zoom ratio of2.4 and an F-number of 2.9. During zooming from the wide-angle end tothe telephoto end, the first lens unit L1 monotonically moves to theimage side, and the second lens unit L2 to the fifth lens unit L5 moveto the object side. During focusing, the second lens unit L2 and thefourth lens unit L4 move.

The zoom lens according to Example 5 is a six-unit zoom lens thatincludes, in order from the object side to the image side, a first lensunit L1 having a negative refractive power, a second lens unit L2 havinga positive refractive power, a third lens unit L3 having a negativerefractive power, a fourth lens unit L4 having a positive refractivepower, a fifth lens unit L5 having a negative refractive power, and asixth lens unit L6 having a positive refractive power. The zoom lensaccording to Example 5 is a zoom lens having a zoom ratio of 2.4 and anF-number of 2.9. During zooming from the wide-angle end to the telephotoend, the first lens unit L1 monotonically moves to the image side, andthe second lens unit L2 to the fifth lens unit L5 move to the objectside. During focusing, the third lens unit L3 and the fifth lens unit L5move.

Numerical examples 1 to 5 corresponding to Examples 1 to 5 will be shownbelow.

In the surface data according to each numerical example, r represents aradius of curvature of each optical surface, and d (mm) represents anaxial distance (distance on the optical axis) between an m-th plane andan (m+1)-th plane, where m is the surface number counted from the lightincident side. nd represents a refractive index of each optical elementfor the d-line, and vd represents an Abbe number of the optical element.An Abbe number vd of a certain material is expressed asvd=(Nd−1)/(NF−NC) where Nd, NF, and NC are refractive indexes for thed-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) in theFraunhofer line.

In each numerical example, a value of each of d, focal length (mm),F-number, and half angle of view (degrees) is set when the zoom lensaccording to each example focuses on the infinity object. A “backfocus”is a distance on the optical axis from the final surface of the lens(closest to the image plane) to the paraxial image plane in terms of theair equivalent length. An “overall lens length” is a length obtained byadding the backfocus to a distance on the optical axis from thefrontmost surface (closest to the object) to the final surface in thezoom lens. The “lens unit” may include one or more lenses.

If the optical surface is an aspherical surface, an asterisk * isattached to the right side of the surface number. The aspherical shapeis expressed as follows:

X=(h ² /R)/[1+{1−(1+k)(h/R)²}^(1/2) +A4×h ⁴ +A6×h ⁶ +A8×h ⁸ +A10×h ¹⁰+A12×h ¹²

where X is a displacement amount from a surface apex in the optical axisdirection, h is a height from the optical axis in the directionorthogonal to the optical axis, R is a paraxial radius of curvature, kis a conical constant, A4, A6, A8, A10, and A12 are the asphericalcoefficients.

“e±XX” in each aspheric al coefficient means “x10^(±XX).”

Numerical Example 1

UNIT: mm Surface data Surface number r d nd vd  1 −428.142 1.75 1.8040046.6  2 37.754 9.55  3 59.622 2.67 1.98612 16.5  4 87.357 (variable)  556.402 1.00 1.68893 31.1  6 28.330 7.81 1.72916 54.7  7 −95.065 0.14  829.059 1.84 1.49700 81.5  9 32.935 (variable) 10 −45.321 1.00 1.6363635.4 11 592.135 (variable) 12 37.952 5.49 2.00069 25.5 13 −136.791 0.5014 (diaphragm) ∞ 2.58 15 −69.090 1.15 1.85478 24.8 16 21.060 6.301.49700 81.5 17 −66.549 0.15 18 50.733 2.72 1.72916 54.7 19 −180.626(variable) 20 −66.439 0.95 1.60342 38.0 21 41.895 (variable) 22* −84.7302.30 1.53110 55.9 23* −10000.000 0.15 24 243.149 2.87 1.95906 17.5 25−108.768 (variable) Image plane ∞ ASPHERIC DATA 22nd surface K =0.00000e+000 A 4 = −1.08643e-004 A 6 = 2.64276e−007 A8 = 5.41174e−011A10 = −9.69154e−012 A12 = 3.59323e−014 23rd surface K = 0.00000e+000 A 4= −9.39877e−005 A 6 = 3.27521e−007 A 8 = −8.60542e−010 A10 =−9.96928e−014 A12 = 4.99140e−015 VARIOUS DATA Zoom ratio 2.35 Wide-angleMiddle Telephoto Focal length  28.84  39.89  67.90 Fno  2.91  2.91  2.91Half angle of view (°)  32.65  28.06  17.67 Image height  21.64  21.64 21.64 Overall lens length 149.46 133.18 115.94 BF  13.48  19.38  27.78d4  54.82  31.15  0.78 d9  8.83  10.86  14.92 d11  7.07  5.04  0.98 d19 2.19  1.51  4.80 d21  12.14  14.31  15.75 d25  13.48  19.38  27.78 ZOOMLENS UNIT DATA Unit Starting surface Focal length 1  1 −59.41 2  5 42.89 3 10 −66.11 4 12  34.55 5 20 −42.44 6 22 149.07

Numerical Example 2

UNIT: mm Surface data Surface number r d nd vd  1 -531.035 1.75 1.7725049.6  2 37.215 8.97  3 55.848 2.67 1.92286 18.9  4 75.855 (variable)  548.808 3.20 1.91082 35.3  6 215.861 (variable)  7 834.033 2.75 1.5934967.0  8 −76.184 (variable)  9 42.644 6.59 1.49700 81.5 10 −32.646 1.151.85478 24.8 11 9510.014 4.24 12 (diaphragm) ∞ 3.41 13 −30.593 1.151.59551 39.2 14 26.646 7.19 1.49700 81.5 15 −42.944 0.15 16 173.357 3.422.00100 29.1 17 −48.634 (variable) 18 9944.835 0.75 1.80610 33.3 1957.905 (variable) 20* −32.063 2.00 1.53110 55.9 21* −348.660 0.15 22161.070 2.41 1.92286 20.9 23 −200.000 (variable) Image plane ∞ ASPHERICDATA 20th surface K = 0.00000e+000 A 4 = −1.19984e−004 A 6 =9.66918e−007 A 8 = −6.32003e−009 A10 = 2.49515e−011 A12 = −3.98821e−01421st surface K = 0.00000e+000 A 4 = −9.93325e−005 A 6 = 8.27469e-007 A 8= −4.58548e−009 A10 = 1.54002e−011 A12 = −2.18663e-014 VARIOUS DATA Zoomratio 2.35 Wide-angle Middle Telephoto Focal length  28.84  39.96  67.90Fno  2.91  2.91  2.91 Half angle of view (°)  32.68  27.27  17.67 Imageheight  18.50  20.60  21.64 Overall lens length 152.91 136.27 115.98 BF 13.50  19.41  26.94 d4  57.92  33.76  0.90 d 6  8.29  6.33  1.87 d 8 3.90  5.86  10.33 d17  2.47  1.63  6.57 d19  14.87  17.32  17.42 d23 13.50  19.41  26.94 ZOOM LENS UNIT DATA Unit Starting surface Focallength 1  1  −58.66 2  5    68.62 3  7   117.75 4  9    51.72 5 18 −72.26 6 20 −229.14

Numerical Example 3

UNIT: mm Surface data Surface number r d nd vd 1 −1261.978 1.75 1.8348142.7 2 29.403 8.02 3 33.753 3.48 1.92286 20.9 4 47.518 (variable) 5435.648 1.30 1.84666 23.8 6 88.348 2.28 1.88300 40.8 7 −113.328 0.96 844.314 2.01 1.77250 49.6 9 193.879 1.64 10 77.542 3.32 1.49700 81.5 11−42.143 1.15 1.74400 44.8 12 133.916 (variable) 13 (diaphragm) ∞ 0.76 1433.973 1.15 1.63980 34.5 15 14.530 6.17 1.49700 81.5 16 −405.427 0.15 1752.432 2.78 2.00100 29.1 18 299.093 (variable) 19 37.718 0.75 1.5481445.8 20 13.807 (variable) 21* −27.130 2.30 1.53110 55.9 22* −24.677 0.1523 ∞ 1.00 1.51633 64.1 24 ∞ (variable) Image plane ∞ ASPHERIC DATA 21stsurface K = 0.00000e+000 A 4 = 1.79444e−005 A 6 = 6.83669e−007 A 8 =−4.55763e−009 A10 = 2.51149e−011 A12 = −4.71818e−014 22nd surface K =0.00000e+000 A 4 = 1.15444e−005 A 6 = 3.82825e−007 A 8 = −1.42844e−009A10 = 1.85804e−012 A12 = 2.26883e−014 VARIOUS DATA Zoom ratio 2.08Wide-angle Middle Telephoto Focal length  28.84  33.20  60.00 Fno  4.12 4.12  4.12 Half angle of view (°)  32.68  31.82  19.83 Image height 18.50  20.60  21.64 Overall lens length 141.33 130.27 104.61 BF  19.70 21.65  35.51 d4  51.44  38.63  3.10 d12  12.65  12.07  6.20 d18  0.99 1.93  5.63 d20  15.44  14.88  13.06 d24  19.70  21.65  35.51 ZOOM LENSUNIT DATA Unit Starting surface Focal length 1  1 −52.26 2  5  50.66 313  39.45 4 19 −40.18 5 21 387.83

Numerical Example 4

UNIT: mm Surface data Surface number r d nd vd  1 −143.018 1.75 1.7725049.6  2 32.312 2.16  3 35.717 4.55 1.84666 23.8  4 60.204 (variable)  548.911 4.77 1.54072 47.2  6 −68.431 (variable)  7 30.129 8.11 1.5952267.7  8 −35.015 1.15 2.00100 29.1  9 219.719 5.51 10 (diaphragm) ∞ 4.7811 −101.380 1.15 1.58267 46.4 12 16.750 9.31 1.49700 81.5 13 −44.2105.92 14 35.210 4.38 1.85026 32.3 15 −59.704 (variable) 16 −515.642 0.751.77250 49.6 17 61.351 (variable) 18* −18.970 1.70 1.90525 35.0 19*−107.214 (variable) Image plane ∞ ASPHERIC DATA 18th surface K =0.00000e+000 A 4 = −5.66177e−005 A 6 = 1.43126e−006 A 8 = −1.27451e−008A10 = 6.21456e−011 A12 = −1.22837e-013 19th surface K = 0.00000e+000 A 4= −4.89520e−005 A 6 = 1.04997e−006 A 8 = −8.61280e−009 A10 =3.71921e−011 A12 = −6.60608e−014 VARIOUS DATA Zoom ratio 2.35 Wide-angleMiddle Telephoto Focal length  28.84  37.16  67.90 Fno  2.91  2.91  2.91Half angle of view (°)  32.68  29.00  17.67 Image height  18.50  20.60 21.64 Overall lens length 121.04 110.42 101.92 BF  9.97  13.25  25.53d4  39.54  24.18  0.87 d6  7.16  7.52  6.80 d15  1.18  1.48  1.15 d17 7.19  7.99  11.56 d19  9.97  13.25  25.53 ZOOM LENS UNIT DATA UnitStarting surface Focal length 1  1 −51.89 2  5   53.52 3  7   27.17 4 16−70.93 5 18 −25.70

Numerical Example 5

UNIT: mm Surface data Surface number r d nd vd 1 −670.741 1.75 1.8040046.6 2 37.100 9.98 3 57.526 2.56 1.98612 16.5 4 80.942 (variable) 555.761 1.00 1.68893 31.1 6 31.691 7.27 1.72916 54.7 7 −89.018 0.12 828.507 1.87 1.49700 81.5 9 32.548 (variable) 10 −45.683 1.00 1.6363635.4 11 193.361 (variable) 12 35.340 5.48 2.00069 25.5 13 −236.182 0.4914 (diaphragm) ∞ 2.34 15 −104.119 1.15 1.85478 24.8 16 19.312 6.731.49700 81.5 17 −66.325 0.15 18 42.684 2.91 1.72916 54.7 19 −223.150(variable) 20 −64.630 0.95 1.60342 38.0 21 35.092 (variable) 22* −93.3652.30 1.53110 55.9 23* −10000.000 0.15 24 155.537 3.00 1.95906 17.5 25−135.592 (variable) Image plane ∞ ASPHERIC DATA 22nd surface K =0.00000e+000 A 4 = −1.03705e−004 A 6 = 1.85435e−007 A 8 = 3.55118e−010A10 = −1.06687e−011 A12 = 3.80401e−014 23rd surface K = 0.00000e+000 A 4= −9.10151e−005 A 6 = 2.50480e−007 A 8 = −4.89160e−010 A10 =−1.47203e−012 A12 = 7.39486e-015 VARIOUS DATA Zoom ratio 2.35 Wide-angleMiddle Telephoto Focal length  28.84  39.87  67.90 Fno  2.91  2.91  2.91Half angle of view (°)  32.65  28.07  17.67 Image height  21.64  21.64 21.64 Overall lens length 146.19 132.32 116.42 BF  14.84  20.01  28.77d4  51.62  30.19  0.78 d9  8.53  10.73  14.84 d11  7.27  5.07  0.95 d19 2.66  1.59  3.99 d21  10.08  13.54  15.88 d25  14.84  20.01  28.77 ZOOMLENS UNIT DATA Unit Starting surface Focal length 1  1 −59.43 2  5 41.65 3 10 −57.97 4 12  31.75 5 20 −37.56 6 22 129.49

Table 1 summarizes various values in each numerical example.

Conditional expression Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 (1) 4.8 < m1 ·f1/skw² < 15.0 10.954 11.893 4.945 9.986 8.034 (2) 2.45 < β2t/β2w < 50.02.673 24.411 3.978 9.294 2.505 (3) 0.05 < TD1/ft < 0.5 0.206 0.197 0.2210.125 0.210 (4) −4.0 < f1/fw < −1.0 −2.060 −2.034 −1.812 −1.799 −2.061(5) 0.8 < f2/fw < 4.0 1.487 2.379 1.757 1.855 1.444 (6) 0.3 < |fr|/fw <20 5.169 7.945 13.447 0.891 4.490 (7) 0.5 < POw/fw < 3.0 1.569 1.5211.664 0.952 1.539 (8) −3.0 < (R2 + R1)/(R2 − R1) < −0.3 −0.838 −0.869−0.954 −0.631 −0.895 (9) 0.2 < m2/fw < 1.4 0.711 0.697 0.403 0.678 0.730(10) −3.0 < fg1/fw < −0.5 −1.494 −1.559 −1.193 −1.178 −1.514 (11) 3.5 <TTDw/skw < 22.0 11.085 11.331 7.174 12.145 9.851 f1 −59.409 −58.656−52.256 −51.890 −59.435 f2 42.895 68.616 50.660 53.515 41.646 f3 −66.115117.754 39.448 27.170 −57.975 f4 34.546 51.717 −40.181 −70.934 31.745 f5−42.440 −72.257 387.825 −25.695 −37.556 f6 149.073 −229.136 — — 129.494fw 28.841 28.840 28.840 28.841 28.842 ft 67.900 67.900 60.000 67.89967.899 β2w −0.497 −1.154 −0.785 −1.235 −0.492 β2t −1.328 −28.172 −3.121−11.479 −1.233 skw 13.484 13.495 19.700 9.966 14.840 m1 −33.523 −36.929−36.722 −19.115 −29.771 m2 20.513 20.092 11.615 19.552 21.065 POw 45.23743.871 47.976 27.453 44.399 TTDw 149.462 152.912 141.331 121.040 146.186TD1 13.969 13.388 13.253 8.464 14.289 R1 −428.142 −531.035 −1261.978−143.018 −670.741 R2 37.754 37.215 29.403 32.312 37.100 fg1 −43.080−44.959 −34.398 −33.972 −43.678

Image Pickup Apparatus

Referring now to FIG. 11, a description will be given of an illustrativedigital still camera (image pickup apparatus) using the zoom lensaccording to the present invention as an image pickup optical system. InFIG. 11, reference numeral 10 denotes a camera body, and referencenumeral 11 denotes an imaging optical system that includes any one ofthe zoom lenses according to Examples 1 to 5. Reference numeral 12denotes a solid-state image sensor (photoelectric conversion element)such as a CCD sensor and a CMOS sensor, which is built in the camerabody 11 and configured to receive an optical image formed by the imagingoptical system 11 and to perform a photoelectric conversion. The camerabody 10 may be a so-called single-lens reflex camera having a quick turnmirror, or a so-called mirrorless camera having no quick turn mirror.

By applying the zoom lens according to each example to an image pickupapparatus such as a digital still camera in this way, the image pickupapparatus can have a small lens.

Each example can provide a compact zoom lens having a large apertureratio and a high optical performance, and an image pickup apparatushaving the same.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2020-085406, filed on May 14, 2020, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A zoom lens comprising, in order from an objectside to an image side, a first lens unit having a negative refractivepower, a second lens unit having a positive refractive power, and threeor more lens units, wherein a distance between adjacent lens unitsvaries during zooming, and wherein the following conditional expressionsare satisfied:4.8<m1·f1/skw ²<15.02.45<β2t/β2w<50.00 where m1 is a moving amount of the first lens unitduring zooming from a wide-angle end to a telephoto end, f1 is a focallength of the first lens unit, skw is a backfocus at the wide-angle end,β2t is a lateral magnification of the second lens unit at the telephotoend, and β2w is a lateral magnification of the second lens unit at thewide-angle end.
 2. The zoom lens according to claim 1, wherein thefollowing conditional expression is satisfied:0.05<TD1/ft<0.50 where TD1 is a distance on an optical axis from a lenssurface closest to an object to a lens surface closest to an image planein the first lens unit, and ft is a focal length of the zoom lens at thetelephoto end.
 3. The zoom lens according to claim 1, wherein thefollowing conditional expression is satisfied:−4.0<f1/fw<−1.0 where fw is a focal length of the zoom lens at thewide-angle end.
 4. The zoom lens according to claim 1, wherein thefollowing conditional expression is satisfied:0.8<f2/fw<4.0 where f2 is a focal length of the second lens unit, and fwis a focal length of the zoom lens at the wide-angle end.
 5. The zoomlens according to claim 1, wherein the following conditional expressionis satisfied:0.3<|fr|/fw<20.0 where fr is a focal length of a lens unit closest to animage plane, fw is a focal length of the zoom lens at the wide-angleend.
 6. The zoom lens according to claim 1, wherein the followingconditional expression is satisfied:0.5<POw/fw<3.0 where POw is a distance from an image plane to an exitpupil position at the wide-angle end, and fw is a focal length of thezoom lens at the wide-angle end.
 7. The zoom lens according to claim 1,wherein the first lens includes a negative lens closest to an object,and wherein the following conditional expression is satisfied:−3.0<(R2+R1)/(R2−R1)<−0.3 where R1 is a radius of curvature of thenegative lens on the object side, and R2 is a radius of curvature of thenegative lens on the image side.
 8. The zoom lens according to claim 1,wherein the following conditional expression is satisfied:0.2<m2/fw<1.4 where m2 is a moving amount of the second lens unit duringzooming from the wide-angle end to the telephoto end, and fw is a focallength of the zoom lens at the wide-angle end.
 9. The zoom lensaccording to claim 1, wherein the following conditional expression issatisfied:−3.0<fg1/fw<−0.5 where fg1 is a focal length of a lens closest to anobject in the zoom lens, and fw is a focal length of the zoom lens atthe wide-angle end.
 10. The zoom lens according to claim 1, wherein thefollowing conditional expression is satisfied:3.5<TTDw/skw<22.0 where TTDw is an overall length of the zoom lens atthe wide-angle end.
 11. The zoom lens according to claim 1, wherein thefirst lens unit consists of a lens having a negative refractive powerand a lens having a positive refractive power arranged in this orderfrom the object side to the image side.
 12. The zoom lens according toclaim 1, further comprising a third lens unit moves during focusing 13.The zoom lens according to claim 1, wherein the three or more lens unitsinclude a third lens unit having a positive refractive power, and afourth lens unit having a positive refractive power.
 14. The zoom lensaccording to claim 1, wherein the three or more lens units include athird lens unit having a positive refractive power, and a fourth lensunit having a negative refractive power.
 15. The zoom lens according toclaim 1, wherein the three or more lens units include a third lens unithaving a positive refractive power, a fourth lens unit having a positiverefractive power, a fifth lens unit having a negative refractive power,and a sixth lens unit having a negative refractive power.
 16. The zoomlens according to claim 1, wherein the three or more lens units includea third lens unit having a negative refractive power, a fourth lens unithaving a positive refractive power, a fifth lens unit having a negativerefractive power, and a sixth lens unit having a positive refractivepower.
 17. The zoom lens according to claim 1, wherein a lens unitclosest to an object has a negative refractive power
 18. The zoom lensaccording to claim 1, wherein the first lens unit monotonically moves tothe image side during zooming from the wide-angle end to the telephotoend.
 19. The zoom lens according to claim 1, wherein at least two of thelens units disposed on the image side of the first lens unit move in thesame trajectory during zooming from the wide-angle end to the telephotoend.
 20. An image pickup apparatus comprising: a zoom lens; An imagesensor configured to receive an formed by the zoom lens, wherein thezoom lens includes, in order from an object side to an image side, afirst lens unit having a negative refractive power, a second lens unithaving a positive refractive power, and three or more lens units,wherein a distance between adjacent lens units varies during zooming,and wherein the following conditional expressions are satisfied:4.8<m1·f1/skw ²<15.02.45<β2t/β2w<50.00 where m1 is a moving amount of the first lens unitduring zooming from a wide-angle end to a telephoto end, f1 is a focallength of the first lens unit, skw is a backfocus at the wide-angle end,β2t is a lateral magnification of the second lens unit at the telephotoend, and β2w is a lateral magnification of the second lens unit at thewide-angle end.